The invention pertains to acoustic imaging methods and apparatus.
Conventional ultrasound imaging produces images that reveal spatial variations in the acoustic impedance of a specimen. In particular, conventional ultrasound images reveal the structure of the specimen based on either the absorption of acoustic waves by the specimen or reflection of acoustic waves in the specimen. The measured reflectances result from abrupt changes in the acoustic impedance of the specimen at material interfaces within the specimen. As one example, in clinical imaging of a human fetus an image of the fetus is obtained based on the difference in acoustic impedance of the fetus and the surrounding amniotic fluid as well as acoustic impedance differences within the fetus.
Because many materials transmit acoustic waves conventional ultrasound imaging provides a method of observing the interior of a variety of specimens, including metallurgical and biological samples. Unfortunately, the conventional ultrasound image depends solely on the acoustic impedance of the specimen and many specimens of interest have a nearly constant acoustic impedance. Thus, the conventional ultrasound images of these specimens lack contrast and reveal little of the specimen""s structure.
Imaging methods have also been described that use an acoustic wave in combination with a specimen property other than the acoustic impedance. For example, thermoacoustic imaging uses optical or microwave radiation to produce localized heating in a specimen. The localized heating depends on the local absorption of the incident radiation and the specimen heat capacity. The localized heating causes a corresponding localized thermal expansion of the specimen that produces an acoustic wave. Images obtained in this way reveal spatial variations in specimen absorptivity, heat capacity, and thermoelastic properties of the specimen.
In another imaging method, a specimen is situated in a magnetic field and an acoustic wave is applied to generate a Hall voltage. The Hall voltage is detected and processed to form an image that reveals localized spatial variations in specimen conductivity. In a variation of this method, an electric field is applied to the specimen, generating an acoustic wave caused by charge movement produced by the Hall effect. The acoustic wave is detected and processed to form an image that depends on the local conductivity.
These methods can provide useful specimen images, but for some specimens, the images have low contrast or fail to reveal important specimen features. In addition, the images produced by these methods may not correspond to specimen properties of interest. For example, conventional ultrasound images of biological specimens primarily reveal density variations in the specimen. While these density variations often produce acceptable images, in many cases specimen properties other than density are important. For example, conventional ultrasound does not reveal specimen ionic properties such as conductivity and mobility, does not distinguish electrolytes from non-electrolytes, and does not distinguish extensively cross-linked materials from more loosely bound materials. Accordingly, improved imaging techniques are needed.
Methods and apparatus for forming images of a specimen based on the electroacoustic properties of the specimen are provided. In addition, methods and apparatus for distinguishing Hall, thermoacoustic, and electroacoustic images are provided.
In an embodiment, an electroacoustic image of a specimen is obtained by applying a probe signal, such as either an acoustic wave or an electric field (or voltage), to the specimen. The probe signal produces an induced signal that is a function of an electroacoustic parameter of the specimen. If the probe signal is an acoustic wave, then the induced signal is an electric field or voltage. If the probe signal is an electric field or voltage, then the induced signal is an acoustic wave. The induced signal is detected and an image is generated based on the detected induced signal.
In one embodiment, the probe signal is an acoustic wave that propagates along an axis of propagation. The electric field is then detected along an axis that is substantially parallel to the axis of propagation. In another embodiment, the electric field is applied parallel to an axis and the acoustic wave is detected in a direction parallel to the axis. These embodiments take advantage of the directional properties of the probe signal and the induced signal.
In some embodiments, a magnetic field is also applied to the specimen. The induced signal is then a function of the applied magnetic field and the image includes an electroacoustic contribution and a Hall effect contribution. Either an acoustic wave or an electric field is applied as the probe signal and produces an induced electric field or acoustic wave, respectively.
In a further embodiment, the probe signal is applied along a plurality of incrementally moved, substantially parallel axes to obtain a two-dimensional image or a three dimensional image of the specimen.
In still another embodiment, a purely electroacoustic image and a purely Hall effect image can be obtained. A first magnetic field is applied to a specimen to obtain a first image of the specimen. The first image then includes both electroacoustic effect and Hall effect contributions. A second magnetic field is then applied in a direction opposite to the direction of the first magnetic and a second image of the specimen is obtained. The second image also includes electroacoustic and Hall effect contributions. A purely electroacoustic image is then obtained by combining the first and second images to substantially eliminate the Hall effect contribution. In addition, the first and second images can be combined in such a way to obtain a purely Hall effect image.
In additional embodiments, thermoacoustic contributions to images can be distinguished. In one method, a first voltage having a first polarity is applied to a specimen to obtain a first image. A second voltage of polarity opposite that of the first voltage is then applied to the specimen to obtain a second image. The first and second images are then combined to obtain a purely electroacoustic image. In addition, the first and second images can be combined to obtain a purely thermoacoustic image.
An apparatus for electroacoustic imaging of a specimen is disclosed that includes an acoustic transducer that produces an acoustic wave in the specimen. The acoustic wave electroacoustically generates an electric field that is detected with electrodes that generate a voltage in response to the electric field. An image processor receives the voltage from the electrodes and produces an image of the specimen on a display. In an embodiment, the acoustic wave has a duration of less than about 1000 ns.
In a further embodiment, an apparatus for forward or reverse electroacoustic imaging of a specimen includes an acoustic transducer situated to transmit an acoustic wave to the specimen or to receive an acoustic wave from the specimen. Two or more electrodes are positioned to apply a voltage to the specimen to electroacoustically generate an acoustic wave or to detect an electroacoustically generated voltage produced by the acoustic wave transmitted by the acoustic transducer. An image processor receives the electroacoustically generated voltage or acoustic wave and forms an image of the specimen.
The disclosed imaging methods and apparatus are suitable for imaging a variety of specimens including, for example, biological specimens, animal tissues, and human bodies, including tumors or human bodily organs, such as the human heart. The methods and apparatus are applicable to both in vivo and in vitro imaging.